This disclosure pertains to, inter alia, devices for use in determining and/or monitoring position of a workpiece in a precision system such as, but not limited to, a system for microlithographically exposing a micro-pattern onto an exposure-sensitive substrate.
Various types of microlithographic exposure systems are currently in use for imprinting micro-patterns onto the surfaces of substrates such as semiconductor wafers. A typical microlithographic exposure system includes an illumination source, a first stage apparatus that holds and positions a pattern master (e.g., a reticle), a second stage apparatus (downstream of the first stage apparatus) that holds and positions the substrate, an imaging optical system situated between the first and second stage apparatus, and a control subsystem connected to and exercising operational control over these apparatus and subsystems. Since the sizes of the pattern elements are very small (now in the several tens of nanometers), the first and second stage apparatus must be capable, as controlled by the control subsystem, of achieving extremely accurate and precise positioning of the stage apparatus and imaging optical system relative to each other so as to achieve corresponding highly accurate exposures.
Substantially all microlithographic exposure systems currently in use employ various sensors, detectors, and other measurement devices for determining and monitoring the accuracy and precision of stage position and of many other operations performed by the exposure system. An example use of sensors and detectors is in devices for performing auto-focus of the imaging performed by the imaging optical system. Auto-focus involves accurate and precise placement of the reticle and substrate relative to the imaging optical system so that exposures made on the wafer have a specified imaging resolution.
For use in auto-focus devices, fluid gauges have been considered for use, either alone or in cooperation with other devices such as slit-projection sensors as described in U.S. Pat. No. 4,650,983. A first conventional example of such a device, called an “air gauge,” is discussed in U.S. Pat. No. 4,953,388, in which the device is configured as a pneumatic bridge. The device includes an air source, from which an air conduit is split to form a measurement arm and a reference arm. Each arm has a respective “probe” from which air is discharged onto a surface. For the measurement arm the surface is that of a workpiece. For the reference aim the surface is part of the gauge and is at a fixed distance from the respective probe. A mass-flow controller is connected between the arms to detect changes in air flow between the two arms resulting from a change in gap distance from the measurement probe to the surface of the workpiece. U.S. Pat. No. 5,540,082 discusses other conventional air gauges used for determining and monitoring position of a workpiece. Both the U.S. Pat. Nos. 5,540,082 and 4,953,388 patent documents are incorporated herein by reference in their respective entireties.
Rather than using a mass-flow controller for determining differential flow of air to the two arms of an air-gauge, a differential pressure sensor can be used. Also, the reference probe can be replaced with a controlled air-bleed device. Changes in gap distance are thus inferred from changes in the mass flow or pressure difference between the measurement and reference arms.
Further disclosures of air gauges and the like are in U.S. Patent Publication No. 2011/0157576 and U.S. patent application Ser. No. 13/753,754, both incorporated herein by reference in their respective entireties to the maximum extent allowed by law.
Air gauges typically respond in a non-linear fashion as the measurement gap changes. This makes calibration of air gauges difficult. In some applications the stage, or the air gauge position, is servoed to keep the measurement gap constant and to minimize any non-linearities. Alternatively, the measurement gap can be servoed to keep the air flows in the measurement and reference arms balanced, so that no change occurs in the differential mass flow or differential pressure. Servoed systems are described in U.S. Pat. Nos. 7,437,911 and 7,797,985, both incorporated herein by reference to the fullest extent allowed by law.
In some applications, it can be difficult with fluid gauges as summarized above to achieve adequate servo-control of the probe(s) due to the complexity of the gauges and of the complexity of the control procedures. For example, complexity is due in part to the fact that the fluid gauge includes at least two air flows (in the measurement arm and at least one reference arm) that must be controlled and/or measured.